Advances in communication services require faster data transmission speeds, which in turn require increasing system bandwidth, improving reception sensitivity, and minimizing interference from other communication systems. Thus, there is a continually growing demand for filters that provide the properties of wider bandwidth, smaller size, lower insertion loss, and higher rejection.
Filters using coaxial resonators are often used, due to the advantages they provide in terms of cost as compared with filters using dielectric resonators such as ceramic filters and monoblock filters. However, while base stations are being built for lower output and smaller sizes, as in the case of small cells, the existing coaxial resonator is limited in terms of how small a size it can have. As such, the use of the smaller-sized dielectric resonator such as the TM mode resonator is gaining popularity, especially for implementing ultra-small filters. However, the dielectric resonator entails the drawback that, because of a difference in thermal expansion coefficients between the dielectric resonator element and the housing, thermal contraction and expansion due to temperature changes can result in the dielectric resonator elements becoming unsecure or providing inadequate contact, which in turn may cause the properties of the filter to change.
Moreover, the coaxial resonator can provide several advantages. For example, the coaxial resonator is advantageous in making small size filters operating at lower frequencies. The coaxial resonator may also improve power handling, which may be important with a small size filter. Here, the air gap between the cover lid and top surface of conventional coaxial resonator may be replaced by a dielectric disk, which may make it easier to control the shift in frequency caused by temperature changes, as a suitable thermal expansion coefficient can be chosen for the dielectric disk. In one example, a modified resonator design for use can include a dielectric disk placed on top of a coaxial resonator made of metal. This would enable smaller filters operate at lower frequencies of below 1 GHz. However, even with this arrangement having a dielectric disk attached to a coaxial resonator, a difference in the thermal coefficients between the dielectric disk and the housing is unavoidable. Thus, expansions and contractions occurring due to changes in temperature may lead to inadequate securing or inadequate contact of the dielectric disk, which in turn may cause changes in the properties of the filter.